Variable-density push-pull sound recording system



July 11, 1950 J. G. FRAYNE I 2,514,633

VARIABLE-DENSITY, PUSH PULL SOUND RECORDING SYSTEM Filed May 7, 1946 2 Sheets-Sheet 1 FIG.

lA/VENTOR J. a. FRAYNE BY X4577? A T TORN Patented July 11, 1950 VARIABIZEPDENSITY PUSH-PULL SOUND RECORDING SYSTEM J ohn: G. Frayne; Pasadena; l. Galifi, assignor toi Western Electric Company; Incorporated;v New York,- N. Y., acorp0ration-of New-York.

Application May 7, 1946; Serial N0. 667,769

4 Claims;

"Ki. I

This-invention' relates to sound film recording and more particularly. to systems for recording'a push-pull va'riabled'e'ansity sound attack on:photo= graphic-film:

It is the object of thisinvention to provide a distortion-freevariable density recording system employing a light valve incorporating a single speech current re'sponsiveribbon.

It isthe usual'pra'ctice in making an original recording of sound accompanying a picture to record the souridthe form of a push-pull, variable density track in which the sound waves in-the'twmseparate tracks aremecorded: 180: degrees out of phase.- The light modulator employed is a four-ribbon light valve of the type disclosed? in: my: United States Pa'tent i 2,199 ;070, issued April 1940. Tliis' four -ribbon=valve is ineffect 'two separatei two-ribbon light valves in which both"signal and biascurrents are applied totheribbons; A four ribbonxlightr. valve requires accurate alignment and almost identical tuning: of four: separate ribbons? Rurtherg. it? is necessary toiconnect the four-ribbon valve to the source ot-sound modulated current through a highimpeda'ncet simplexcircuitvwhich produces a; relatively high resonance -peakiiofv the valve. Also; the bias-current traversing the ribbons .produces a heating effect which adversely affects the tuning frequency. of. the ribbons.-

Itlwas recognizedby applicant that. the .abovementioned'i disadvantages" incident to the use of a four-ribbon light valve couldbe avoided'inrecording a push-pull variable density sound track by utilizing a modulator'in the formof a light valve incorporating" but one'speech currentresponsive ribbon, theoppOsitee'dgesof which are adapted to modulate the separate light beams in a push-pullma'nner. Noise. reduction could be obtained if desirediby. incorporating. into the modulator two ribbons electrically isolated from the'singlespeech ribbon; the two ribbons serving to define the: fixed' edge r of 'eacht recording light beam and movable in' accordance with the impressed noise reduction currents. However, while the suggested simplification: of." the modulating device or lightvalve avoids the disadvantages incident to the-'u'se of a four-ribbon light valve, even order harmonic distortion will be introduced dueto the fact thatthe' proposed light valve 'is ess'entially two single ribbon light valves each-of which, per se, has-ahigh second harmonic introduced by the well-known ribbon velocity effect.

In a light'valve of the t'ypediscussed above the modulating edgesof the signal ribbon" are in opposite positions with: respect' 'tol thesepa' rat'el l'ight beams; The instantaneous relative motion: of i thesignaliribbom edges stcnthe is in thesame direction in both cases which brings about a condition where the even harmonics? of the two stacks thus exposed" are additive. Also the centerlinesiof thei exposed area ofthe two tracks. are only in line for zero modulation and move apart with increasing-- modulation. It. will be -obvious' that to be :a lpractical push-pull @modula'tor the proposed: simplified light valve must becapablezof recording on amoving-filma sound track in which'the even harmonicsr may be can'- cele'd out: in reproduction.

Applicant conceived theiideathata distortion free, push pullf variable density sound track couldaibe successfullyrecorded' on a movingfilm by a single= ribbon vlight-valve by the utilization of optical means between the lighttvalve and film producing an: inversion of one ofthe ribbon modulated lightibeams'; By so inverting one of the modulated lightrbeams; thermodulating: edges of the signal ribbon move inopposite directions atany instant with'respect to thefilm motion for: the two'component tracks, thus producing a. condition where the. even harmonics are completelycanceled' by reason of the'fa'ct that these components from" the two 1ight beamsare' recorded in opposite phase relative to their respective fundamentals.- By the use of additional optical='means=in.the form of'refractor plates, commonlysknown as sawbucks, the two images may be-transversely aligned at the film.

The invention may be more readily" understood by. reference-tether following; specification when read in connection-withthe accompanying drawing; in which:

Figlis aplan view; partly in-section, of the recording optical-system in. accordance with this invention;

Fig: 21s a: side elevation of'the' optical systemshowninFig. 1;

Fig. 3 is an enlarged elevation of thepole face ofone aperturedpole-piece of a'lightvalve with a signal: modulated ribbon disposed in front thereof;

Fig. 4- is similar to Fig; 3 but shows. an arrangement of a signal modulated ribbon and-two noise reduotionribbonsdisposed in front of the aper tured polepiece Fig 5' is an enlarged diagrammatic showing ofthe" im'ageof the light beamsonthefilm as defin'edby the light valve ribbonszinvanoptical system in which" neither an inverting. prism: nor refracto'r. plate is: employed;

Fig. 6 is an enlarged showing of the images of the light beams at the film in a system employing only refractor plates for transversely aligning the images; and

Fig. 7 is an enlarged showing of the images of the light beams at the film when both refractor plates and optical inverter are employed in accordance with the invention.

Referring to Fig. 1, light rays of constant intensity from a light source I are projected by means of a condensing lens system 2 to focus at a point between pole-pieces 3 and 4 of a light valve. The remaining structure of the light valve is not shown but, as is well known, it comprises either a permanent magnet or an electromagnet producing a magnetic field in the air-gap between the opposed faces of the pole-pieces 3 and 4. The light rays emerging from the light valve are projected by means of an objective lens system 5 to a moving film F, supported in the optical system by a rotatable drum 6.

The pole-piece 3 is provided with two transversely separated, longitudinally ofifset recording apertures A and B. Pole-piece 4 is provided with similar offset apertures aligned with the apertures A and B in pole-piece 3.

The constant intensity light beam from source I, which. passes through the aligned apertures in pole-pieces 3 and 4, is modulated by means of vibrating ribbon l in a push-pull manner. Referring to Fig. 3, it will be seen that the upper edge of ribbon l, which vibrates in the magnetic field in response to speech currents passing therethrough, moves the position of the lower edge of the light beam formed by aperture A while the lower edge of ribbon 1 moves the position of the upper edge of the light beam formed by aperture B. The ribbon 1 is electrically connected to a source of modulated currents by leads 9 and i0.

Ribbons II and I2, which are adapted to carry the bias current, are mounted in the air-gap in a separate but parallel plane to the plane of ribbon 'l, as shown in Figs. 2 and 4. As shown in Fig. 4, ribbon ll moves the position of the upper edge of the light beam formed by aperture A while ribbon l2 moves the lower edge of the light beam formed by aperture B. The bias current travels in opposite directions in ribbons H and i2 thus causing them to move toward and away from each other in accordance with variations in bias current. Ribbons H and I2 are shunted by a potentiometer l3 which is used to correct for any variation in sensitivity of either noise reduction ribbon, the bias currents in each ribbon being adjusted in order to obtain equal biased spacing for each aperture.

As pointed out above, Fig. 5 discloses diagrammatically an enlarged view of the images at film F of the apertures defined by the light valve ribbons for a recording system thus far described. Movement of ribbon 1 of Fig. 3 or 4 will produce simultaneous movement of the modulated edge of the image of Fig. 5 in the same direction at any instant relative to the motion of the film. The opposite edges of each image for the single ribbon valve of Fig. 3 will remain fixed. For the light valve as shown in Fig. 4, the opposite edge of each image as shown in Fig. 5 will move in opposite directions in accordance with the bias currents. In this case we have the maximum displacement of the centers of the exposed areas for zero modulation and the distance between the centers increases as the modulation is increased. It can be shown that while the exposure of each sound track is 180 degrees out of phase in respect to each other, no cancelation of the even harmonics results when the two tracks are reproduced in push-pull. This is because the even harmonics introduced by the ribbon velocity effect in each sound track are recorded in the same phase relative to their respective fundamentals and direct addition of the even harmonics occurs in the same way as for the fundamentals.

Referring back to Figs. 1 and 2, refractor plates or sawbucks l5 and I6 are disposed one in each light beam for the purpose of transversely aligning the images of the offset apertures at the film F. Optical elements of this type are disclosed and described in my above-mentioned United States Patent 2,199,070.

Fig. 6 discloses diagrammatically an enlarged view of the images at film F of the apertures defined by the light valve ribbons for a recording system as described above but including the refractor plates l5 and Hi. In this case the noise reduction edge of one image is aligned with the modulating edge of the second image. The images will only remain aligned for the unbiased condition, becoming more and more out of alignment as the bias current is increased. In this case the same condition prevails as in Fig. 5, that is, the even harmonics introduced by ribbon velocity effect in each sound track are recorded in the same phase relative to their respective fundamentals and are, therefore, additive in reproduction.

In accordance with this invention, a recording optical system is provided in which the single speech current ribbon light valve may be successfully employed in recording a push-pull variable density sound track in which the even harmonics produced by the ribbon velocity effect are recorded in each track in opposite phase relative to their respective fundamentals and are thus completely canceled out when both tracks are scanned in pushpull in reproduction.

This is accomplished in accordance with the invention by the provision of an inverting prism i! in the path of one of the light beams formed by the apertures in the light valve. As shown in Figs. 1 and 2, the inverting prism I1 is placed in the path of the light beam formed by aperture B. The passage of light from aperture B through inverting prism I1 is compensated by the addition of a rectangular prism l8 in the path of the light beam formed by aperture A.

Referring to Fig. '7, it will be seen that the image of the aperture B at the film F has been inverted thus producing transverse alignment of the fixed or noise reduction edges and transverse alignment of the modulating edges of the image for zero modulation. The centers of the images of the apertures A and B will stay in alignment at the film plane irrespective of the amount of bias applied to ribbons H and I2. The motions of the modulating edges of the image are in opposite directions at any instant thus producing a condition where the even harmonics produced by ribbon velocity effect in each track are recorded in opposite phase relative to their respective fundamentals. Therefore, in push-pull reproduction of the two tracks these even harmonic distortions are canceled out.

In accordance with this invention there is, therefore, provided a push-pull variable density recording system employing a simplified modulator or light valve incorporating but one speech responsive ribbon which must be tuned to a relatively high frequency. If noise reduction ribhome are incorporated they can be tuned to a much lower frequency, thereby making a considerable saving in noise reduction current. The electrical separation of signal and noise reduction circuits eliminates the necessity of a simple coupling circuit thus further reducing the noise reduction current normally required.

What is claimed is:

1. A variable density, push-pull recording SETS- tem comprising in combination, a moving film, a source of modulated current, means producing a pair of longitudinall ofiset light beams, optical means for projecting said pair of light beams to adjacent transverse areas of said film, a conducting ribbon disposed in a magnetic field and electrically connected to said source of current to produce movement of said conductor to simultaneously vary the width of each of said light beams from those transverse edges of said beams which are nearest to each other in a direction along the longitudinal axis of said film, the direction of motion of the width varying edge of each beam being in the same direction, means for transversely aligning said light beams at said film and means for inverting one of said light beams whereby the images of the width varying edges of said light beams move in opposite directions at the film in accordance with said current.

2. A variable density, push-pull recording system comprising in combination, a moving film, a source of sound modulated current, a source of current varying in accordance with the envelope of the amplitude variations of said modulated current, means producing a pair of longitudinally offset light beams, optical means for projecting said pair of light beams to adjacent transverse areas of said film, a conducting ribbon disposed in a magnetic field and electrically connected to said source of modulated current to produce movement of said conductor to simultaneously vary the width of each of said light beams from those transverse edges of said beams which are nearest to each other in a direction along the longitudinal axis of said film, the direction of motion of the width varying edge of each beam being in the same direction, a pair of conducting ribbons disposed on opposite sides of and in parallel relation with said first-mentioned ribbon, said pair of conducting ribbons being electrically connected to said source of envelope currents to produce simultaneous movement thereof in opposite directions to vary the width of said light beams from an edge opposite the first-mentioned edge of each of said light beams, means for transversely aligning said light beams at said film and means for inverting one of said light beams with respect to its initial position.

3. A variable density, push-pull recording system comprising in combination, a moving film, a source of modulated current, means producing a pairof longitudinally offset light beams, optical means for projecting said pair of light beams to adjacent transverse areas of said film, a single means movable in response to said current to simultaneously vary the width of each of said beams from those transverse edges of said beams which are nearest to each other, the direction of motion of the width varying edge of each beam being in the same direction, means for transversely aligning said light beams at said film plane and means for inverting one of said light beams whereby the images of the width varying edges of said light beams move in opposite directions at the film in accordance with said current.

4. A variable density, push-pull recording system comprising in combination, a moving film, a source of sound modulated current, a source of current varying in accordance with the envelope of the amplitude variation of said modulated current, means producing a pair of longitudinally ofiset light beams, optical means for projecting said pair of light beams to adjacent transverse areas of said film, a single element movable in response to said sound modulated current to simultaneously vary the width of each of said beams from those transverse edges of said beams which are nearest to each other in a direction along the longitudinal axis of said film, the direction of motion of the width varying edge of each beam being in the same direction, a pair of elements movable simultaneously in opposite directions in response to said envelope currents to simultaneously vary the width of said light beams from an edge opposite the first-mentioned edge of each of said light beams, means for transversely aligning said beams at said film and means for inverting one of said light beams with respect to its initial position.

JOHN G. FRAYNE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,031,835 Kellog Feb. 25, 1936 2,077,193 Wente Apr. 13, 1937 2,147,623 Wender Feb. 14, 1939 2,161,369 McLeod June 6, 1939 2,275,537 Manderfeld Mar. 10, 1942 2,305,875 Kellog Dec. 22, 1942 2,318,138 Benfer May 4, 1943 2,445,832 Kellogg July 27, 1948 

